A Proposed Test Methodology for Evaluating the Electrostatic Characteristics of Floor Materials

Because the movement of people and materials through sensitive electronics environments generate static charges, the selection and use of floor materials to control electrostatic discharge becomes a critical issue. Frequently, however, users of
these materials base their evaluation, comparison, and subsequent selection on a potpourri of electrostatic properties and test procedures, many of them either inappropriate or inadequate. Facing the same ill-define requirements,
material vendors are at a loss to provide adequate products information for decision making and have little guidance for developing and offering products to meet use requirements.

Current Evaluation Criteria

Traditionally, vendors and users rely upon three approaches for evaluating floor materials. First, electrical resistance that indicates electrical continuity across a surface or from surface to ground, but does not necessarily indicate performance properties. Second, body voltage generation as measured according to AATCC/ANSI 134, a procedure developed for less critical computer and consumer environments. Third, material decay testing which is applicable to packaging materials, but does not indicate a floor material’s ability to dissipate a charge from an object or person in contact with it. Other authors have discussed the evaluation of ESD floor materials, but either did not include the human body in the test procedure 1, 2 or limited their work to very specific materials8,9 or to specific applications10.

This paper proposes that a full and thorough characterization of the electrostatic properties of ESD floor materials include the following characteristics: electrical resistance, body voltage generation, body voltage decay, voltage gradient, and voltage suppression. The test methodology for measuring these properties has been adapter from the previously described procedures and specifically modified for the analysis of floor materials used in highly sensitive environments. Where appropriate, the procedures include the human body for measurements of voltage generation and decay, and they simulate the conditions under which the floor materials are likely to be used. Because static generation and dissipation characteristics are a function of the interaction between floor and footwear, the methodology includes six types of footwear most likely worn in conjunction with the floor materials. It applies to a variety of materials such epoxy floor coatings and resilient floor coverings with and without floor finishes.

TEST METHODOLOGY – GENERAL

Samples

Floor test bed: A test bed of commercial vinyl composition tile installed on concrete below grade and finished with standard commercial floor finish is used as a reference point. It is considered an “uncontrolled” floor material.

Flooring materials: 4’x4’ samples of each floor covering are mounted according to the manufacturer’s recommendations on plywood or pressboard. If a conductive adhesive is recommended for installation, that adhesive is used to mount the samples. The 4’x4’ sample provides and adequate area in which to perform the walking pattern required in the body voltage generation procedure.

Floor finishes: Samples of the floor covering on which the floor finish is used are mounted on 4’x4’ pieces of plywood or pressboard. The floor finish is then applied in accordance with the manufacturer’s instructions.

Footwear: the body voltage generation and decay test use six different types of footwear, representing the range of footwear common in the manufacturing environment. Prior to testing, resistance to ground plate measurements are made of the footwear.

Shoes with insulative soles (Rubber or other synthetic soles)

Shoes with insulative soles and conductive heel strap.

Tennis style shoes with static dissipative soles.

Deck style shoes with static dissipative soles.

Dress shoes with leather soles.

Oxford styles shoes with conductive soles (ANSI Z-41).

Preparation and Conditioning of Samples: Prior to testing, each floor material samples is cleaned twice with a solution of 70% alcohol and water. Floor finishes are simply wiped with a dry, low-linting cloth. Shoe samples are prepared by stroking the soles six times with 1—grit sandpaper, then cleaned twice with a solutions of 70% alcohol and water. All samples are air dried and conditioned at least 48 hours at 20-24% RH and 68-72°F.

TEST METHODOLOGY – ELECTRICAL RESISTANCE

Although not a predictor of voltage generation, resistance indicates a path of electrical continuity that may allow the flow of static charges across the surface or from surface to ground. This procedure adapts the general accepted procedures of ASTM F150 and NFA 99, but modified the electrodes and the applied voltages. In addition to the traditional foil-covered 5 pound, 2½” diameter cylindrical metal electrodes, the procedure includes a second configuration that substitutes a conductive elastomer for the foil and non-conductive rubber contact surface. The procedure supplements the standard ohmmeter prescribes by ASTM F150 and NFPA 99 with a precision Dr. Theidig Milli-TO wide range ohmmeter.

Fig 1: Resistance-Surface to Surface Fig 2: Resistance to Ground

Procedures

After conditioning, the individual floor sample is placed on the test bed and lightly wiped with a low linting cloth. The contact surfaces of the electrodes are cleaned with a solution of 70% alcohol and water. For point to point measurements (Rtt), the electrodes are placed 36” apart on the surface of the test sample (Fig. 1). For electrodes to groundable point measurements (Rtg), one terminal of the ohmmeter is connected to the groundable point of the test sample, and the other terminal is connected to an electrodes placed on the surface of the sample (Fig. 2). The groundable point is defined as “The connection, such as a ground bolt, snap, wire, or conductive adhesive that is used to attach a floor material to an appropriate ground…”13.

The test voltage is applied and readings recorded 5 seconds after application of the voltage. Measurements are made at applied voltages of 10V, 100V and 500V and with both the foil-covered electrodes and the conductive elastomer-covered electrodes. Five measurements are taken and the results averaged for each test condition. The samples are tested in an ungrounded condition; however, they are temporarily grounded between measurements.

TEST METHODOLOGY – BODY VOLTAGE GENERATION

The methodology combines two procedures for evaluating body voltage generation in a test subject wearing various types of footwear. The first adapts the AATCC/ANSI 134 method and measures the maximum body voltage during a defined walking cycle. The second measures the minimum voltage obtained at the conclusion of the walking cycle with both feet placed on the floor. These two procedures are conducted and recorded simultaneously. The two procedures illustrate the effects of capacitance, which is directly related to whether or not both feet are on the test surface. The formula for parallel plate capacitance is:

where,

C = Capacitance

k= Constant (Permittivity)

A = Area of shoe surface

d= Distance between sole of foot and floor surface

At the end of the walking cycle, both shoes are on the floor specimen, shoe surface area (A) is maximum, and distance from the floor (d) is minimum. Therefore, capacitance (C) is maximum with both feet on the floor and capacitance (C) is minimum where one or both shoes are removed from the floor surface during the walking cycle.

The second aspect of this relationship considers the primary electrostatic relationship of charge, capacitance, and voltage:

where,

Q = CHARGE

C = Capacitance

V = Voltage

As capacitance (C) increases, voltage (V) decreases. Therefore, with both feet on the floor, capacitance (C) is at a maximum and measures voltage (V) will be at a minimum. With one foot raised from the floor surface, capacitance (C) decreases and measured voltage (V) increases. Thus, the two procedures provide a range of minimum to maximum voltages that would be expected with the floor and footwear combination.

In addition to these basic voltage measurements, the procedure includes a statistical analysis that determines the probability of equaling or exceeding a defined body voltage.

After preconditioning, the sample’s groundable point is connected to the constantly monitored earth ground. The test operator places the wrist strap on the wrist and connects the discharge end to the charge plate monitor. The operator walks on the floor sample in a six-step pattern and comes to rest with both feet on the floor specimen in front of the PVT. The step pattern required forward and backward steps and a cross over step when changing movement directions (Fig 4).

Two sets of measurements are obtained during the procedure. The first measures voltage generation during the walking cycle and is defined as the Maximum Body Voltage. The second measures the voltage obtained at the end of the cycle with both feet on the floor specimen and is defined as the Minimum Body Voltage.

During the walking cycle, the XY plotter records all operator body voltages as seen through the wrist strap by the Simco EA-1. At the end of the pattern with both feet flat on the floor specimen, the operator touches the PVT device to record and enter the body voltage into the PVT memory. A test assistant also marks the XY plotter chart to indicate the moment of PVT measurement. The process is repeated until all footwear and floor material samples have been tested.

The average and standard deviation (n-1) of the 20 highest peak walking voltages from the XY recorder are entered into the electrostatic analyzer. A normal (Gaussian) probability distribution is assumed for determining the probability of equaling or exceeding a defined body voltage. For purposes of comparing various floor and footwear combinations the statistical analysis determines the body voltage above which there is less than 0.1% probability of being exceeded.

The probability function (f(N)) of the Gaussian distribution is given as:

and is pictures as the normal bell-shaped curve.

TEST METHODOLOGY – BODY VOLTAGE DECAY

The body voltage decay procedure measures the time required for a charge to dissipate from the body through a given floor material and footwear combination. This combination should dissipate any accumulated charge from the body before exposure to sensitive parts can occur.

The test subject wears the test footwear and wrist strap and stands motionless on the insulated acrylic plate next to the floor specimen. The wrist strap is connected to the charge plate monitor. The subject is charged to =/- 5,000V while standing on the acrylic plate. When the charging voltage is turned off, the subject steps from the plate to the floor specimen. As the foot makes initial contact with the floor, the decay is tracked on the XY plotter until the voltage reaches zero or no further decay occurs. This procedure is performed six times at +5,000V and six times at -5,000V for each floor material and footwear combination.

TEST METHODOLOGY – VOLTAGE GRADIENT

Voltage gradient measurements indicate the practical dissipative characteristics of a floor material. Originally developed by G. Baumgartner to evaluate ESD work surfaces, the procedure has been modified for use on floor materials. The method measures the voltage “seen” by an object resting on the surface when a second nearby object is charged to 2,000V. This measurement indicated whether dissipation is across the material’s surface or through its bulk to ground or both. If the path is across the surface, tote boxes, carts, or other objects on the floor can be exposed to the charge as dissipates.

Procedures

An NFPA metal electrode (sensing electrode) is placed near the material’s installed groundable point and attached to a metal plate isolated from ground. The plate voltage is measured with a non-contacting, precision electrostatic voltmeter (Monroe Model #244) connected to an XY plotter (Hewlett Packard #7105B). A second electrode (powered electrode) is placed on a diagonal line from one (grounded) corner to the opposite corner. Attached to this second electrode is a current-limited 2KV power supply (ETS #810). With the floor material connected to earth ground, 2KV is applied to the powered electrode and the maximum voltage indicated on the sensing electrode is recorder through the electrostatic voltmeter (Fig. 6).

One measurement is made with the sensing electrode 5” from the material’s groundable point and the powered electrode 5” from the sensing electrode. A second measurement is made with the sensing electrode 12” from the mater’s groundable point and the powered electrode 36” from the sensing electrode (Fig. 7). Six measurements at +2,000V and at -2,000V are made for each electrode position. If more than one groundable point is provided on the test specimen, the test is repeated for each groundable point.

This test procedure was developed by Dr. Joe Crowley in conjunction with preliminary work for the EOS/ESD Association Draft Standard 4 on work surfaces. The test indicated a material’s ability to drain a charge from a charged object such as a person wearing specialized footwear. As the object contacts an ESD floor material, the charge should fully drain.

Procedures

A round aluminum test plate, ⅛” thick by 6” in diameter with an attached insulated handle is connected to the plate of a charge plate monitor (Simco EA-1). A Hewlett Packard #7105B XY plotter is also connected to the charge plate monitor to record the test voltages. With the floor sample grounded, the test plate is charged to +/-5,000V and lowered onto the sample’s surface for approximately 2 seconds. The test plate is carefully tilted at an angle, then lifter free of the work surface (Fig. 8). Residual voltage, if any, remaining on the aluminum plate is recorded by the plotter. Any charge generated by lifting the plate free of the surface is also recorded. Six +5,000V and six -5,000V test cycles are conducted at different positions on the specimen.

Fig. 8: Test Instrumentation (Voltage Suppression)

OBSERVATIONS AND DISCUSSION

Electrical Resistance

This paper covers testing performed on six different ESD floor materials. Data from these materials (Floors A, B, C, D, E, F and G) and the uncontrolled test bed (Floor C) are presented in Table I , II, III, and IV.

A basic material attribute, electrical resistance is not a measure of either indicated or actual performance. The resistance measurements do, however, characterize basic differences in material properties across a broad range of materials (Table 1). Floors A and E represent typical conductive floor materials with resistances in the 104 – 106 range. Floors B, D, F, and G represent typical dissipative floor materials with resistances in the 106 – 1010 range. The test bed (Floor C) measured greater than 1012.

As expected, the level of applied voltage had a strong effect on the measured resistance of the sampled with the lower voltages resulting in higher resistance values. Similarly, the conductive elastomer electrode surfaces tended to give lower resistance values than the foil-covered electrodes. The measurements made at lower voltages and with the conductive elastomer electrodes also tended to show less variability and better reproducibility. These observations are consistent with earlier work on worksurfaces4 and floor materials 5 for EOS/ESD Association standard sub-committees. Based largely on this earlier work, it appears that the use of conductive elastomer electrode surfaces and applied voltages of 10V or 100V are the preferred alternative for measuring the resistance of floor materials.

Body Voltage Generation and Decay

Unlike resistance, body voltage generation and decay represent actual performance characteristic of the floor materials. In a controlled test environment, the proposed procedures provide indicators of material performance in actual use. These two parameters are the two most critical characteristics on which to evaluate the performance of ESD floor materials.

The test data indicate significant differences in body voltage generation and decay characteristics among the various ESD floor material and footwear combinations (Table II) and provide several insights into material differences. First, as expected voltage generation and decay vary among floor materials with given footwear as shown in Figures 9 and 10. However, the decay rates for the test bed floor material (Floor C) with controlled footwear may be partially suppressive effect rather than total decay.

Second, body voltage generation is not a function of material resistance. Floor G is three orders of magnitude higher in resistance that Floors A and E, yet Floor G generates significantly lower body voltage (Fig. 11). Floors B, D, F and G are similar in resistance, but significantly different in voltage generation (Fig. 12).

Third, voltage generation and decay times vary with the footwear. For example, Figures 13 and 14 illustrate the differences in voltage generation and decay time for Floor D with different footwear. Not surprisingly, voltage generation levels and decay times were the highest with uncontrolled footwear (insulated or leather soles) and lowest with the conductive footwear. In environments where devices with ESD sensitivities less than 200 volts are handled, insulated shoes soles in combination with any of the floor materials tested would results in unacceptable levels of static generation. Leather soles house also showed problems with most of the floor materials.

Fourth, a probability analysis can help predict the performances of various floor and footwear combinations to aid in material selection. For example, the combination of dissipative floor material D and dissipative tennis shoes has < 0.1% probability of generating a charge in excess of 70V with both feet on the floor such as might occur at a work station (Fig. 15). The same combination has <0.1% probability of generating in excess of 450V while a person walks through the work area (Fig. 16). Other combinations of the same floor material with different footwear show different probabilities of specific body voltages being generated as shown in Figures 17 and 18. Depending upon the environments requirements, these other combinations might be appropriate. Or, other floor materials in combinations with various footwear can be evaluated (Table II).

Voltage Suppression

Voltage suppression characteristics of floor materials provide additional data on which to evaluate the various alternatives. Although there are no specific criteria on which to base an analysis of this characteristic, the various materials tested here exhibit significant differences in voltage suppression. Two of the specimens, Floors A and B, Showed no suppressive effects at all (Table IV). The charge on the metal plate was fully dissipated by contact with the floor material, and no residual charge remained when the plate was removed. The other four materials that were tested (Floors C, D, E, and G) showed residual voltages of varying magnitude. Floors E and G had previously shown the best generation and decay characteristics, but retained residual voltages of greater than 100V.

Voltage Gradient

The final parameter of evaluation is voltage gradient and again it appears that resistance is not the determinant of the differences between materials (Tables I and III). It is more likely that material design and installation are the influencing factors. Those materials, such as Floor G, that have a conductive pathway across their surface rather than a conductive pathway into the bulk of the material, such as Floor A, are more likely to show higher voltage levels because the current travels across the surface to the sensing electrode. In establishing the qualification criteria for evaluating floor materials on this characteristic, one has to determine whether the environment require protection from exposure to voltages that may move across the surface of the floor material. In some situations where unprotected pars are sored or laid upon the floor, this can be an important parameter.

Body voltage generation and decay are critical parameters of evaluation, but the levels of performance depend upon the combination of floor material and footwear.

The proper footwear is critical to the performance of ESD floors.

Performance on one parameter is not necessarily and indicator of performance on others.

Evaluation of ESD floor surfaces should be performed on parameters other than electrical resistance. The test methodology proposed here allows evaluation across a wide range of electrostatic properties: static generation, static decay, voltage suppression, voltage gradient and electrical resistance. The procedures are designed to specifically evaluate and compare flooring materials which are intended for very sensitive electronic manufacturing environments. They include both footwear and the human body which are encountered in the environment and the testing is conducted in a manner similar to the end use of the products in questions. Combining the performance characteristics of the materials with appropriate statistical analysis can lead to a prediction of performance in the field and can lead to better selection of the most appropriate floor material and footwear combinations for the specified application.